A problem in wireless communications is processing symbols received via a time dispersive channel. A time dispersive channel causes interference between symbols due to a spreading of the symbol energy in the time domain. The delay spread of a channel is indicative of the degree of channel dispersion. Also, transmitting symbols at higher data rates generally requires symbol duration and distance between the symbols to be reduced. Thus, the higher the data rate, the closer together the symbols, and the stronger the interference between symbols, in many transmission schemes.
Orthogonal Frequency Division Multiplexing (OFDM) is an effective modulation technique for reducing such interferences. OFDM operates by transmitting blocks of multiple symbols in parallel over a channel. This technique results in much larger OFDM symbols being transmitted over a particular period, relative to other methods. Thus, because the symbol duration and the distance between symbols can be longer than the delay spread, the potential for overlap between symbols due to the dispersive channel is reduced. An example is a wireless personal area network (WPAN) designed according to the MBOA (WiMedia Multiband OFDM Alliance) PHY (physical layer) specifications v1.0, incorporated herein by reference, the current highest data rate of which is 480 Mbps. The delay spread of the channel can be several tens of nanoseconds. By grouping 100 symbols into one OFDM symbol having a duration of 312.5 nanoseconds and defining appropriate guard times between OFDM symbols, the effect of symbol interferences is reduced using these specifications.
More specifically, to further reduce OFDM symbol interferences, the OFDM symbols can be sent through multiple frequency bands (e.g., the MultiBand-OFDM transceiver detailed in the WiMedia MBOA PHY specifications v1.0 uses three sub-bands). Therefore, the distance between the OFDM symbols in one band can be increased to provide an additional protection against OFDM symbol interference, while maintaining or increasing the symbol transmission rate.
However, proper decoding of the received OFDM symbols requires synchronization (i.e., a correct estimation of the arrival time at the receiver of the OFDM symbols). Inaccurate synchronization results in inter-carrier interference (ICI). During the decoding of an OFDM symbol, ICI results in a loss of orthogonality between the multiple symbols making up the one OFDM symbol.
To reduce ICI, currently the orthogonality of the symbols is maintained by either preprocessing the blocks of symbols in a transmitter using a cyclic prefix, or post-processing the symbols using a zero padding technique. The zero padding technique operates by adding a tail of an OFDM symbol created by a time dispersive channel to the beginning of the OFDM symbol, as is described in more detail in the WiMedia MBOA PHY specifications.
While the general processing of OFDM symbols in the receiver is specified by the WiMedia OFDM PHY specifications, one challenging part of receiver design is selecting an optimal timing for processing and demodulating transmitted OFDM symbols. Also, the optimal timing depends on the type of preprocessing that was performed in the transmitter.
What is needed, therefore, are synchronization techniques for reducing the effects of time dispersive wireless communications channels.